Various hybrid powertrain architectures are known for managing the input and output torques of various prime-movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to a battery pack. The internal combustion engine in a series hybrid is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel hybrid architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation.
It is known in the art of vehicular powertrain controls to interpret an operator's request for torque into a system torque command to affect an output torque to the vehicle driveline. Such interpretation and command require relatively simple control management dominated by the available engine torque in relation to a vehicle's present set of operating parameters, which relationship is relatively well understood. In electrically variable transmission based hybrid powertrains a number of factors in addition to the available engine torque affect the output torque that can be provided to the vehicle driveline. It is known in such hybrid powertrains to interpret an operator's request for torque into a system torque command and allow individual sub-system limitations to dictate actual output torque. Such limitations include, for example, available engine torque, available electric machine torque and the available electrical energy storage system power. It is preferable to understand the various subsystem individual and interactive constraints affecting available powertrain output torque such that output torque commands are issued consistent with such torque availability and subsystem constraints.
In the case of the electric machine torque, the limitations that affect the output torque that can be provided to the vehicle driveline include the maximum and minimum torque output limits of the electric machine or machines. During vehicle operation and the real-time control of the vehicle powertrain and EVT, these maximum and minimum torque limits are typically used in conjunction with the determination of the available operation or control points of the EVT, including the control of a number of transmission control parameters, such as the input speed, output speed, input torque and output torque. There are various operating conditions wherein the electric motor or motors may be operating at or near the maximum or minimum, such that the desired motor output torque (and the contribution to the transmission output torque) may be constrained, thereby affecting the desired control of the vehicle. For example, for synchronous shifts, the potential exists for a sudden change in output torque command at first synchronization because of a system output torque constraint change, such as a targeted input acceleration change; which may result in unacceptable input speed control if the system happens to be operating at or near one or more of the maximum or minimum motor torque limits. Such limits can thus affect the shift synchronization and overall shifting performance. In another example, if the measured input speed begins to vary from the desired input speed, either higher or lower, and the electric machine or machines happen to be being operated at or near their maximum or minimum limits, the ability to use them to address the input speed control problem may be restricted or prohibited altogether. This limitation may necessitate sudden and undesirable changes in transmission output torque or other transmission control parameters to affect the necessary input speed control.
Therefore, it is desirable to establish a real time reserve of motor torque to avoid vehicle operation at control points that do not permit a smooth or robust response of the vehicle powertrain using the electric machine or machines.